Confused about tri-gum printing/filtering in camera

Hi there!
I'm looking into gum bichromate printing specifically for the tri-printing (gum over cyanotype) look. I stupidly looked it up on wikipedia and now i'm confused because it says

Printing in three colors

1. Make three color separations (blue, green, and red filters) use panchromatic film
2. Mix three pigmented gum solutions (yellow, magenta, and cyan) add sensitizer before applying each to the paper
3. Coat with sensitized yellow gum and expose to the blue separation.
4. Process and dry, recoat with magenta gum to print to the green separation.
5. Repeat in cyan and red separation.

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I'm confused because I thought I could do Red, Green, Blue filtration ... then print in Red, Green, and Blue (cyanotype). Why are they crossing the blue separation with yellow? I know this is some sort of simple science thing I didn't get to in school but... I really seriously thought that a red separated negative would be printed as the red layer :confused:

I'm completely putting the cart before the horse here as I've not tried gum printing before nor even seen one in real life but I'd like to know what I should be doing before hand. Still a little nervous about the potassium dichromate aspect of it as it sounds like pretty nasty stuff.

Hi Heather,
It seems confusing at first, but it does make sense. The blue separation prints the yellow layer, the red separation prints the cyan layer, the green separation prints the magenta layer. If you're interested in an explanation of how this is so, see my page here:

http://www.pacifier.com/~kthayer/html/RGBseps.html

This explanation assumes starting from a positive image, so the channels must be inverted to produce negatives; this wouldn't be true of your in-camera separations which would start as negatives.

Katharine

Akki14 said:

Still a little nervous about the potassium dichromate aspect of it as it sounds like pretty nasty stuff.

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P.S. I really wouldn't worry too much about the dichromate. Don't drink it, don't eat it, don't inhale the powder, and you should be fine. It's not going to hurt you just being around it, in the small amounts we use. The one thing to be aware of is that some people develop a contact dermatitis on their skin over time if they put their ungloved hands into the developing bath; in order to avoid that it might be good to have a habit of wearing gloves, but I've never wanted to bother with putting gloves on and taking them off as I go back and forth from coating and printing to checking prints in the water, so I've never taken that precaution, and I've never had a problem (knock on wood).

Katharine Thayer said:

P.S. I really wouldn't worry too much about the dichromate. Don't drink it, don't eat it, don't snort the powder, and you should be fine. It's not going to hurt you just being around it, in the small amounts we use. The one thing to be aware of is that some people develop a contact dermatitis on their skin over time if they put their ungloved hands into the developing bath; in order to avoid that it might be good to have a habit of wearing gloves, but I've never wanted to bother with putting gloves on and taking them off as I go back and forth from coating and printing to checking prints in the water, so I've never taken that precaution, and I've never had a problem (knock on wood).

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Its a potential carcinogen - Use it (I still do for three processes) but wear gloves and dont breathe the dust ...

Its amazing how clingy chemicals can be and how they transfer from one surface to the next so I suppose its inevitable you'll ingest some, just try to minimize it.

I just came out of 8 months chemotherapy - so within reason its better to be safe than sorry

nick mulder said:

Its a potential carcinogen - Use it (I still do for three processes) but wear gloves and dont breathe the dust ...

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I agree, it's better to be safe than sorry, and yes, it's a known carcinogen, but the cancers that have been linked to it have been primarily cancers of the breathing passages in workers who were exposed to large amounts of airborne material for hours every day for years, such as the guys who used to spray the chome on chrome bumpers for cars (thank goodness cars don't have chrome bumpers any more, so no one has to be exposed to that risk any more). My point is that as long as you take some care, which after all is what I was recommending, there's no reason to be terrified of it in the tiny amounts that we use for gum printing.

Heather,

Consider the effect of shooting a landscape with a yellow taxi in the picture, bright blue sky, and green, grassy meadow. Since yellow is the opposite (negative) color to blue, if you were to use a blue filter on the camera and monochrome negative film, the light from the blue sky would be passed by the filter and expose more relative to the green and yellow objects. Yellow light would be blocked by the filter and so the yellow cab would be underexposed and thin on the negative, and the green grass would be somewhere in between.

Now print this monochrome separation negative using a sensitized gum layer containing yellow pigment. Where the blue sky has exposed the negative, the negative density is great and little/no printing light gets through to expose the yellow gum layer. Where the negative is thin (the taxi area), a lot of light gets through to expose the yellow layer. The green density is in between and receives a moderate exposure of the yellow layer.

When the print is processed, the unhardened/underexposed pigmented gum layer washes away. Since the sky area let no light expose the yellow layer, the yellow gum all washes away there. The taxi area let a lot of light through to harden the gum and so yellow is deposited there. Again, a moderate amount of yellow is trapped where the green grass built up a moderate density. So the blue negative/yellow print layer essentially has no yellow in the sky, a little in the grass, and a lot in the taxi.

Now when the red negative is printed with cyan pigment, the situation is similar but with a different color choice. Cyan is printed to the sky heavily because the red filter has blocked the cooler wavelengths (and the negative is thus thin in the sky area and allows a relatively great exposure of the cyan pigment layer), and less so to the grass, and very little if any to the taxi area. In combination on the print, the sky is approaching blue, the taxi is still yellow, and the grass greener.

Printing the green camera filter/magenta pigment layer adds the final color components in the same manner. As the layers are sequentially printed with transparent pigments, the layers add together to produce the full-color rendition. (Which is not to say the result will accurate color. It will vary depending on pigment choice and relative exposures both in the camera and the print.)

Hope this explanation makes things a bit clearer.

I always wear gloves for all my photographic stuff, even if they're just simple "marigold" yellow washingup type gloves. I was just wondering how best to minimise powder going everywhere as I don't have a fume cupboard/hood setup at home. I'd probably be trying to minimise contact by doing the weighing of powder and mixing with water in my bathtub (well not a bathtub amount obviously but it's an easier surface to wash down and should mostly limit any clothing exposure too when I kneel down). I have safety goggles too just not a mask yet (not sure what to buy here in the UK). Is potassium dichromate very powdery like flour (or ammonium ferric citrate) or is it more like salt crystals (or more accurately like potassium ferricyanide since I'm familiar with that)? Just trying to get a sense of how much of a problem dust is going to be.

I just had a silly idea of putting plastic wrap or a clear plastic dust sheet on top of the tub and stick my gloved hands under that into the bathtub and work that way to minimise dust. I hope washing down the tub with water won't spread dust around too much though.

smieglitz - Ahh now I understand, I see that if I printed yellow with a yellow filter it'd just wash away. I'll probably still have to write down in big letters the filter to printing colours to remind myself though.

Akki14 said:

Is potassium dichromate very powdery like flour (or ammonium ferric citrate) or is it more like salt crystals (or more accurately like potassium ferricyanide since I'm familiar with that)?

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It usually arrives as crystals but due to internal friction/wear there will always be an amount of dust 'at the bottom' - think of a packet of crisps/chips or breakfast cereal - just like that but on a smaller scale...

A very breathable dust can whip up in no time flat once you pull a spoon of crystals out, often hard to see ...

Akki14 said:

smieglitz - Ahh now I understand, I see that if I printed yellow with a yellow filter it'd just wash away. I'll probably still have to write down in big letters the filter to printing colours to remind myself though.

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Heather, I still mark each of the separations with letters showing both the separation color and the printing color (B>Y) even after I've printed tricolor gum for almost 20 years.

By the way, Joe, that was a great explanation.

The only time the dust is likely to be a problem is when you're mixing up new dichromate solution. I've been told that to be on the safe side you should wear a mask when weighing and mixing dichromate, but I never have. If I'm going to get cancer from some environmental cause, it's more likely to be from science projects and experiments I did as a young science student, back before we understood the dangers of things like unshielded x-rays, radioactive iodine, benzene's ability to soak right through the skin (I used to put my hands in it; we all did), than from a little dichromate. But I do agree with the advice that it's always better to be safe than sorry, as I've said.

Funny, on another forum I've often been the one advising more caution, but there what I'm usually responding to are suggestions that dichromate is essentially harmless, even perfectly safe to eat, which of course it's not. I'd recommend taking a middle course: treat it with respect, but don't lose any sleep over it either.
kt

Joe,
That was the most succint, direct, and clearest explanation of the tri-gum printing process I've ever come across. Certainly far better than any book I've read on the topic. Your students are so lucky to have a teacher who can demistify such complex material.